Active matrix type display apparatus

ABSTRACT

A plurality of display pixels PX arranged in a matrix form respectively have a driving transistor which controls an electric current amount made to flow in a self-luminescent element, in accordance with an image signal, a first switch formed of a transistor and connected between a gate and a drain of the driving transistor, and a second switch formed of a transistor and connected between the driving transistor and the self-luminescent element. A first capacitance Cs is provided between the gate and a source of the driving transistor, and a second capacitance Cx is provided between the second switch and the gate of the driving transistor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Continuation Application of PCT application No.PCT/JP2004/006925, filed May 14, 2004.

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2003-139440, filed May 16,2003, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an active matrix type display apparatuswherein a display screen is configured by arranging display pixelsincluding self-luminescent elements such as, for example, anelectroluminescence (hereinafter, referred as EL) element in a matrixform.

2. Description of the Related Art

Flat panel type display apparatuses have been broadly used as a displayapparatus for a personal computer, an personal digital assistant, atelevision, or the like. In recent years, as such a flat panel typedisplay apparatus, an active matrix type organic EL display apparatususing a self-luminescent element such as an organic EL element has beengiven the attention, and the research and development thereof have beenactively carried out. The organic EL display apparatus has the followingfeatures: it does not require a backlight preventing the organic ELdisplay apparatus from being made to be thin and light-weight, it has ahigh-speed responsiveness and is suitable for playing-back movingpicture, and moreover, it can be used at cold districts as well, becausethe brightness thereof is not reduced at a low temperature.

Generally, the organic EL display apparatus comprises a plurality ofdisplay pixels which are arranged in a plural rows and a plural columnsto constitute a display screen, a plurality of scanning lines extendingalong the respective rows of the display pixels, a plurality of signalconductor lines extending along the respective columns of the displaypixels, a scanning line driving circuit for driving the respectivescanning lines, a signal conductor line driving circuit for driving therespective signal conductor lines, and the like. Each display pixelincludes an organic EL element which is a self-luminescent element, anda pixel circuit for supplying a driving electric current to the organicEL element. The pixel circuit has a pixel switch disposed in thevicinity of the cross positions of the scanning lines and the signalconductor lines, a driving transistor which is connected in series tothe organic EL element between a pair of power source lines, and whichis formed of a thin-film transistor, and a storage capacitance retainingthe gate control voltage of the driving transistor. The pixel switch ismade to be conductive in response to a scanning signal supplied from acorresponding scanning line, and acquires an image signal supplied froma corresponding signal conductor line into the pixel circuit. The imagesignal is written as the gate control voltage into the storagecapacitance, and is stored for a predetermined period. The drivingtransistor supplies an electric current amount corresponding to the gatecontrol voltage written in the storage capacitance to the organic ELelement, and the organic EL element is operated to emit light.

The organic EL element has a cathode, an anode, and an emitting layerwhich is formed of a thin-film including a fluorescent organic compoundand provided between the cathode and the anode. The organic EL elementgenerates an exciton by injecting electrons and holes into the emittinglayer and recombining those, and emits light due to the light emissiongenerated at the time of deactivation of the exciton. The organic ELelement emits light at a brightness corresponding to a supplied electriccurrent amount, and a brightness of about 100 to 100,000 cd/m² can beobtained by even an applied voltage equal to or less than 10 V.

In the organic EL display apparatus, a thin-film transistor serving asthe driving transistor has a semiconductor thin-film formed on aninsulating substrate such as a glass. Therefore, the characteristics ofthe driving transistor such as a threshold voltage Vth and a carriermobility μ depend on the manufacturing process or the like, and easilyvary. If there is unevenness in the threshold voltage Vth of the drivingtransistor, it is difficult to make the organic EL element emit light atan appropriate brightness. Thus, an irregularity in brightness among theplurality of display pixels arises, which causes unevenness indisplaying.

For example, in U.S. Pat. No. 6,229,506, there is disclosed a displayapparatus in which threshold canceling circuits are provided at all ofdisplay pixels in order to avoid the effect due to the irregularity inthe threshold voltage Vth. Each threshold canceling circuit isconfigured such that the control voltage of the driving transistor isinitialized by a reset signal supplied in advance of an image signalfrom the signal conductor line driving circuit. Further, as the otherdisplay apparatus, in U.S. Pat. No. 6,373,454, there is proposed adisplay apparatus in which writing of an image signal is carried out byan electric current signal, and an attempt is made to uniform thebrightness of light emission by reducing the effect due to theirregularity in the threshold voltage in the driving transistor.

In the display apparatus described above, the pixel circuit of eachdisplay pixel has one or more switches which are connected between agate and a drain of the driving transistor, and which are in OFF-statesfor a period of light-emitting, and the switches are respectively formedof thin-film transistors. However, in such a pixel circuit, when theseswitches are switched from being on to being off, feedthrough voltagesdue to the parasitic capacitance formed between the gates and thesources of the switches are generated. Further, a gate control voltageof the driving transistor is varied by an amount corresponding to theamount of the generated feedthrough voltages. Because the feedthroughvoltage depends on a threshold voltage of the switch, an irregularityarises in the gate control voltage of the driving transistor due to theirregularity in the threshold voltage, and an irregularity in brightnessarises among the plurality of display pixels. Such an irregularity inbrightness among the display pixels appears as the unevenness indisplaying, which deteriorates the quality of displaying.

For example, when the above-described switches and the drivingtransistor are formed of P-channel type thin-film transistors, the gatecontrol voltage of the driving transistor is varied in plus electricpotential direction, and electric current made to flow in the drivingtransistor changes for being reduced. This leads to a reduction in ELlight emitting current, which decreases white brightness on a displayedimage.

By supplying from the driving circuit an image signal to which an amountof the reduction in light emitting current is added in advance, theproblem of the insufficient white brightness can be avoided. However, inthis case, a rise in a driving voltage of the driving circuit, upsizingof the driving circuit, an increase in manufacturing cost, or the likeare brought about.

BRIEF SUMMARY OF THE INVENTION

The present invention has been contrived in consideration of theabove-described problems, and its object is to provide an active matrixtype display apparatus in which variation in the electric potential of adriving transistor due to a feedthrough voltage is compensated for, andthe quality of displaying is improved.

In order to achieve the object, an active matrix type display apparatusaccording to an aspect of the present invention comprises: aself-luminescent element which is connected to a first voltage powersource line and which emits light in accordance with a supplied electriccurrent; a driving transistor which is connected between a secondvoltage power source line and the self-luminescent element and whichcontrols an electric current amount supplied to the self-luminescentelement in accordance with a gate control voltage; a first switch formedof a transistor and connected between a gate and a drain of the drivingtransistor; a first capacitance connected to the gate of the drivingtransistor; a second switch formed of a transistor and connected betweenthe drain of the driving transistor and the self-luminescent element;and a second capacitance connected between the second switch and thegate of the driving transistor.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate an embodiment of the invention,and together with the general description given above and the detaileddescription of the embodiment given below, serve to explain theprinciples of the invention.

FIG. 1 is a circuit diagram illustrating a configuration of an organicEL display apparatus according to a first embodiment of the presentinvention.

FIG. 2 is a diagram illustrating an equivalent circuit of display pixelsin the organic EL display apparatus.

FIG. 3 is a plan view schematically illustrating the display pixel.

FIG. 4 is a cross sectional view illustrating a part of the organic ELdisplay apparatus.

FIG. 5 is a timing chart for explanation of the operation of the displaypixel shown in FIG. 2.

FIG. 6 is a plan view schematically illustrating the display pixel in anorganic EL display apparatus according to a second embodiment of thepresent invention.

FIG. 7 is a diagram illustrating an equivalent circuit of display pixelsin an organic EL display apparatus according to a third embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

An active matrix type organic EL display apparatus according to a firstembodiment of the present invention will be described in detail withreference to the accompanying drawings.

As shown in FIG. 1, the organic EL display apparatus has an organic ELpanel 10 and a controller 12 for controlling the organic EL panel 10.

The organic EL panel 10 has m×n of display pixels PX which are arrangedin a matrix form on a light transmittable insulating substrate 8 such asa glass plate or the like, and which configure a display region 11,first scanning lines Y (Y1 to Ym) and second scanning lines Bg (Bg1 toBgm) which are connected to the respective rows of the display pixelsand are independently provided by m lines, n of signal conductor lines X(X1 to Xn) connected to the respective columns of the display pixels, ascanning line driving circuit 14 for successively driving the first andsecond scanning lines Y and Bg for each row of the display pixels, and asignal conductor line driving circuit 15 for driving the signalconductor lines X1 to Xn. The driving circuits 14 and 15 are provided onthe insulating substrate 8.

Each display pixel PX includes an organic EL element 16 serving as aself-luminescent element, and a pixel circuit 18 for supplying a drivingelectric current to the organic EL element. An equivalent circuit of thedisplay pixels is shown in FIG. 2, and one example of a planar structureis shown in FIG. 3. The pixel circuit 18 is a current signal systempixel circuit for controlling the light emission of the organic ELelement 16 in accordance with an image signal formed of an electriccurrent signal, and has a pixel switch 20, a driving transistor 22, afirst switch 24, a second switch 26, and a first capacitance Csfunctioning as a storage capacitance and a second capacitance Cx. Thepixel switch 20, the driving transistor 22, the first switch 24, and thesecond switch 26 are configured of the same conductivity typetransistors, for example, P-channel type thin-film transistors.

The driving transistor 22, the second switch 26 and the organic ELelement 16 are connected in series between a first voltage power sourceline Vss and a second voltage power source line Vdd. The source of thedriving transistor 22 is connected to the second voltage power sourceline Vdd. One electrode, i.g., a cathode, of the organic EL element 16is connected to the first voltage power source line Vss. The source ofthe second switch 26 is connected to the drain of the driving transistor22, and the drain thereof is connected to the anode of the organic ELelement 16. Moreover, the gate the second switch 26 is connected to thesecond scanning line Bg. The first and second voltage power source linesVss and Vdd are respectively set to, for example, the electricpotentials of 0 V and +10 V.

The driving transistor 22 outputs signal electric current correspondingto an image signal to the organic EL element 16. The second switch 26 iscontrolled to be turned on (in a state of being conductive) and off (ina state of being nonconductive) by a control signal Sb from the secondscanning line Bg, and controls the connection and non-connection betweenthe driving transistor 22 and the organic EL element 16.

The first capacitance Cs is connected between the source and the gate ofthe driving transistor 22, and retains the gate control electricpotential of the driving transistor 22 determined by the image signal.The first capacitance Cs has a pair of tabular electrodes facing eachother in parallel, and is formed as a parallel plate type capacitance bya gate electrode film and a polysilicon layer of the driving transistor.

The pixel switch 20 is connected between the signal conductor line Xcorresponding thereto and the drain of the driving transistor 22, andthe gate thereof is connected to the first scanning line Y. The pixelswitch 20 acquires an image signal from the corresponding signalconductor line X in response to a control signal Sa supplied from thefirst scanning line Y.

The first switch 24 is connected between the drain and the gate of thedriving transistor 22, and the gate thereof is connected to the firstscanning line Y. The first switch 24 is turned on and off in accordancewith the control signal Sa from the first scanning line Y, and controlsthe connection and non-connection between the gate and the drain of thedriving transistor 22. In FIG. 2, Cgs denotes a parasitic capacitancegenerated between the gate and the source of the first switch 24.

The second capacitance Cx is connected between the source of the secondswitch 26 and the gate of the driving transistor 22. The secondcapacitance Cx has a pair of tabular electrodes facing each other inparallel, and is formed as a parallel plate type capacitance. Thecapacitance value of the second capacitance Cx can be adjusted by anarea of the capacitance, and a concrete capacitance value will bedescribed later.

In the present embodiment, all of the thin-film transistors constitutingthe pixel circuits 18 are formed by the same process, have the samelayer structure, and are thin-film transistors having a top gatestructure using polysilicon as the semiconductor layers. Due to all ofthe pixel circuits 18 being configured of the same conductivity typethin-film transistors, an increase in the number of manufacturingprocesses can be suppressed. The second switch 26 may be formed of aconductivity type thin-film transistor which is different from the pixelswitch and the first switch, i.e., an N-channel type thin-filmtransistor. In this case, control of the gate of the second switch 26may be carried out by the same signal which is used for the control ofthe pixel switch 20, thorough the first scanning line Y.

Next, the configurations of the pixel circuit 18 and the organic ELelement 16 will be described in detail with reference to FIGS. 3 and 4.FIG. 4 illustrates one example of the structure, in particular, of thesecond switch 26, the second capacitance Cx, the driving transistor 22,the first capacitance Cs, and the organic EL element 16 in the pixelcircuit 18.

The P-channel type thin-film transistor constituting the second switch26 has a semiconductor layer 50 formed on the light transmittableinsulating substrate 8 and formed of polysilicon. The semiconductorlayer 50 has a source region 50 a, a drain region 50 b, and a cannelregion 50 c positioned between the source region and the drain region. Agate insulating film 52 is formed on the semiconductor layer 50, and agate electrode G is provided on the gate insulating film so as to facethe channel region 50 c. An interlayer insulating film 54 is formed onthe gate electrode G, and a source electrode (source) S and a drainelectrode (drain) D are provided on the interlayer insulating film. Thesource electrode S and the drain electrode D are respectively connectedto the source region 50 a and the drain region 50 b of the semiconductorlayer 50 via contacts which extend passing through the interlayerinsulating film 54 and the gate insulating film 52. The respectivethin-film transistors configuring the first switch 24, the pixel switch20, and the driving transistor 22 are formed so as to have the samestructure described above.

A plurality of wirings such as the signal line X, the second voltagepower source line Vdd, and the like are provided on the interlayerinsulating film 54. On the interlayer insulating film 54, a passivationfilm 56 is formed so as to cover the source electrode S, the drainelectrode D, and the wirings. A hydrophilic film 58, a partition film 60are successively laminated on the passivation film 56.

The organic EL element 16 has an anode 62, a cathode 66 and an organicemitting layer 64 including luminescence organic compound and interposedbetween the anode 62 and the cathode 66. The anode 62 is formed of atransference electrode material such as ITO (indium tin oxide) or thelike, and is provided on the passivation film 56. Those portions of thehydrophilic film 58 and the partition film 60 which are located on theanode 62 have been eliminated by etching. Further, an anode buffer layer63 and an organic luminescent layer 64 are formed on the anode 62. Thecathode 66 formed of, for example, barium-aluminum alloy is laminated onthe organic luminescent layer 64 and the partition film 60.

In the organic EL element 16 having such a structure, when a holeinjected from the anode 62 and an electron injected from the cathode 66are recombined at the inside of the organic emitting layer 64, anexciton is generated by exciting organic molecules configuring theorganic emitting layer. Light is generated in the process ofdeactivation of the radiation of the exciton, and the light is emittedto the exterior via the transparence anode 62 and the lighttransmittable insulating substrate 8 from the organic emitting layer 64.

In the above embodiment, the anode 62 is connected to the drain of thedriving transistor 22, and the cathode 66 is connected to the firstvoltage power source line Vss. However, the cathode 66 may be connectedto the drain of the driving transistor 22, and the anode 62 may beconnected to the first voltage power source line Vss.

In the above-described embodiment, the substrate 8 side on which theorganic EL element 16 is formed is the display surface. However, theside (the cathode 66 side in the above-described embodiment) facing thesubstrate 8 on which the organic EL element 16 has been formed may bethe display surface.

In any case, the light emitting surface side must be formed of atransparent conductive material. For example, when the cathode 66 isdisposed on the light emitting side, the formation can be achieved byforming alkaline earth metal or rare earth metal so as to be thin to anextent of having a light permeability.

The controller 12 shown in FIG. 1 is formed on a printed circuit boarddisposed at the outside of the organic EL panel 10, and controls thescanning line driving circuit 14 and the signal conductor line drivingcircuit 15. The controller 12 receives a digital image signal and asynchronization signal supplied from an external device, generates avertical scanning control signal for controlling the timing of verticalscanning and a horizontal scanning control signal for controlling thetiming of horizontal scanning on the basis of the synchronizationsignal, and supplies the vertical scanning control signal and thehorizontal scanning control signal respectively to the scanning linedriving circuit 14 and the signal conductor line driving circuit 15.Further, the controller 12 supplies the digital image signal to thesignal conductor line driving circuit 15 synchronously with thehorizontal and vertical timings.

The signal conductor line driving circuit 15 converts image signalsData1 to Datam successively obtained at respective horizontal scanningperiods by the control of the horizontal scanning control signal intoanalog formats, and supplies the analog signals as electric currentsignals to the plurality of signal conductor lines X in parallel. Thescanning line driving circuit 14 includes a shift register, an outputbuffer, and the like, and successively transfers horizontal scanningstart pulses supplied from the exterior, to the next stage, and suppliestwo types of control signals, i.e., the control signal Sa and thecontrol signal Sb to the display pixels PX at the respective rows viathe output buffer. In accordance therewith, the respective first andsecond scanning lines Y and Bg are respectively driven by the controlsignal Sa and the control signal Sb at the first horizontal scanningperiods different from one another.

The operation of the pixel circuit 18 based on the output signals fromthe scanning line driving circuit 14 and the signal conductor linedriving circuit 15 will be described with reference to a timing chartshown in FIG. 5.

The scanning line driving circuit 14 generates a pulse having widths(Tw-Starta) corresponding to the respective horizontal scanning periodson the basis of, for example, a start signal a (Starta) and a clock a(Clka), and outputs the pulse as the control signal Sa. Further, thescanning line driving circuit 14 generates the control signal Sb byinverting the control signal Sa.

The operation of the pixel circuit 18 can be divided into an imagesignal writing operation and a light emitting operation. At a point intime t1 of FIG. 5, when the control signals Sa and Sb for turning thepixel switch 20 and the first switch 24 on (in a state of beingconductive) and turning the second switch 26 off (in a state of beingnonconductive), i.e., the control signal Sa which is at a low level andthe control signal Sb which is at a high level here, are outputted, theimage signal writing operation is started. For an image signal writingperiod (t1 to t2), the driving transistor 22 is in a state of diodeconnection, and the image signal Data is acquired from the correspondingsignal conductor line X via the pixel switch 20. By the source ofconstant current outputting a value of the constant currentcorresponding to an image signal, the electric current made to flowbetween the source and the drain of the driving transistor 22 is set tothe value of constant current. In accordance therewith, image signalwriting is carried out, and an electric potential between the gate andthe source of the driving transistor in which the electric currentamount can be made to flow is written into the first capacitance Cs.

Next, at a point in time t2, the control signal Sa and the controlsignal Sb are respectively made to be at a high level and at a lowlevel, and the pixel switch 20 and the first switch 24 are turned off,and the second switch 26 is turned on. In accordance therewith, theimage signal writing operation is completed, and light emittingoperation is started. For the light-emitting period, the drivingtransistor 22 is turned on by the gate control voltage written in thefirst capacitance Cs, and supplies an electric current amountcorresponding to the image signal to the organic EL element 16. Thus,the organic EL element 16 emits light, and the light emitting operationis started. The organic EL element 16 maintains the light emitting stateuntil the time when the control signal Sa is supplied again after aperiod of one frame.

At the time of completing the writing period, when the first switch 24is switched from the ON-state to the OFF-state, a feedthrough voltagedue to the parasitic capacitance Cgs of the first switch is generated,and is supplied to the first capacitance Cs. Accordingly, the electricpotential of the first capacitance Cs, i.e., the gate electric potentialof the driving transistor 22 is displaced in plus direction. In contrastthereto, after the first switch 24 is made to be in the OFF-state, whenthe second switch 26 is switched from the OFF-state to the ON-state, thesource electric potential of the second switch is displaced in minuselectric potential direction. This electric potential displacement inminus direction is transmitted to the gate of the driving transistor 22through the second capacitance Cx provided between the source of thesecond switch 26 and the gate of the driving transistor 22. Inaccordance therewith, the plus displacement of the gate electricpotential generated at the time of switching the first switch 24 off iscompensated for by the minus displacement generated at the source of thesecond switch 26, and the amount of the variation in the gate electricpotential of the driving transistor 22 can be compensated for.

In order to compensate for the variation in the gate control voltage, itis preferable that the second capacitance Cx has such a capacitancevalue that the sum of the plus displacement and the minus displacementof the gate electric potential described above is made to be zero. Thecapacitance value of the second capacitance Cx can be adjusted bycontrolling an area of the capacitance, and is set in consideration ofthe parasitic capacitance Cgs, the first capacitance Cs, an electricpotential difference between the ON-state and OFF-state electricpotentials of the control signal Sa, of the first switch 24, or thelike.

For example, given that Cgs=0.01 pF, Cs=1 pF, and an electric potentialdifference ΔVs1, between the ON-state and OFF-state electric potentialsof first switch 24, =15 V, a gate electric potential displacement amountΔVg1 of the driving transistor 22 which is generated when the firstswitch 24 is switched off, i.e., a generated feedthrough voltage isapproximated byΔVg1={Cgs/(Cgs+Cs)}×ΔVs1,and it follows that ΔVg1=150 mV.

On the other hand, a gate electric potential displacement amount ΔVg2 ofthe driving transistor 22 when the second switch 26 is turned on dependson a source electric potential displacement amount ΔVs2 and the secondcapacitance Cx of the second switch, and is approximated byΔVg2={Cx/(Cx+Cs)}×ΔVs2.

The source electric potential displacement amount ΔVs2 of the secondswitch 26 is expressed by a difference between the drain electricpotential of the driving transistor 22 at the time of image signalwriting operation and the anode potential of the organic EL element 16at the time of light emitting operation, and it is, for example, about−5 V.

Provided that the second capacitance Cx which results in ΔVg1+ΔVg2=0 isdetermined by using these values, it follows 0.03 pF, and it sufficesthat the area of the capacitance is determined on the basis of thevalue. For example, in the capacitance with an oxide film whose filmthickness is 100 nm being used as a dielectric, an area of thecapacitance corresponding to 0.03 pF is 9×9 μm=81 μm². If the secondcapacitance Cx has such an area, there are few constraints on an area,when the second capacitance is added to the display pixel Px.

As described above, in the organic EL display apparatus according to thepresent embodiment, the second capacitance Cx is provided between thesource of the second switch 26 and the gate of the driving transistor22, and the variation in the gate electric potential of the drivingtransistor generated at the time of switching the first switch 24 offcan be compensated for. Therefore, while reliably carrying out the imagesignal writing operation, the influence of the variation or irregularityin the gate control voltage of the driving transistor caused by afeedthrough voltage can be reduced, and the irregularity in brightnessamong a plurality of display pixels can be suppressed. Further, there isno need to supply an excessive electric current amount from the drivingcircuit for the purpose of compensating the variation in the gatecontrol voltage, and sufficient white brightness can be obtained withoutincreasing the driving voltage and the size of the driving circuit, andincreasing in manufacturing cost thereof. Accordingly, there can beprovided an active matrix type organic EL display apparatus in whichunevenness in displaying is prevented, and the quality of displaying isimproved.

As shown in the above-described embodiment, it is preferable that thesecond capacitance Cx is set to be ΔVg1+ΔVg2=0. However, the presentinvention is not limited thereto. Provided that the second capacitanceCx has a capacitance value compensating for at least some of thevariation in the gate electric potential generated at the time ofswitching the first switch 24 off, the effect on the reduction of thevariation in the gate control voltage can be obtained.

The parallel plate type capacitance is used as the second capacitanceCx. However, a parallel inter-wiring capacitance may be used. Forexample, when a luminous efficiency of the organic EL element is high,or when high white brightness of the display screen is not required, avoltage for the light emission of the organic EL element and a voltagefor driving a pixel circuit in the organic EL display apparatus can bemade to be low. As one example, given that the electric potentialdifference ΔVs1 between the ON-state electric potential and theOFF-state electric potential of the first switch 24 is 10 V, and theother conditions are the same as the embodiment described above, itfollows that a required capacitance value of the second capacitance Cxis 0.02 pF. As shown in FIG. 6, the second capacitance Cx having such acapacitance value can be formed by a parallel inter-wiring capacitance.Namely, the second capacitance Cx has at least a pair of wirings, andthese wirings are arranged in parallel with a predetermined gap withoutsuperposing onto each other.

When the parallel inter-wiring capacitance is used as the secondcapacitance Cx, there can be obtained the advantage in whichlayer-to-layer short can be exactly avoided as compared with the case ofthe parallel plate type capacitance. For example, it is possible toconstitute a capacitance of 0.02 pF by using parallel wirings whosespace is about 1 μm and whose parallel length is about 80 μm. In thesecond embodiment shown in FIG. 6, the configurations other than thesecond capacitance Cx are the same as the first embodiment, and the sameportions as in the first embodiment are denoted by the same referencenumerals, and detailed descriptions thereof will be omitted.

The pixel circuit of the organic EL display apparatus may be structuredas, not only the electric current signal system pixel circuit, but alsoa voltage signal system pixel circuit. FIG. 7 illustrates display pixelsPX of an organic EL display apparatus according to a third embodiment ofthe present invention. Each display pixel PX includes an organic ELelement 16 which is a self-luminescent element, and a pixel circuit 18for supplying a driving electric current to the organic EL element. Thepixel circuit 18 is a voltage signal system pixel circuit forcontrolling the light emission of the organic EL element 16 inaccordance with an image signal formed of a voltage signal, and has apixel switch 20, a driving transistor 22, a first switch 24, a secondswitch 26, a first capacitance Cs1, a second capacitance Cx, and a thirdcapacitance Cs2. The pixel switch 20, the driving transistor 22, thefirst switch 24, and the second switch 26 are configured of the sameconductivity type transistors, for example, P-channel type thin-filmtransistors.

The source of the driving transistor 22 is connected to a high potentialsecond voltage power source line Vdd. A first capacitance Cs1 isconnected between the gate and the source of the driving transistor 22,and the first switch 24 is connected between the gate and the drain ofthe driving transistor 22. The gate of the driving transistor 22 isconnected to the drain of the pixel switch 20 via the third capacitanceCs2, and the source of the pixel switch is connected to a signalconductor line X.

The drain of the driving transistor 22 is connected to the source of thesecond switch 26, and the drain of the second switch is connected to theanode of the organic EL element 16. The cathode of the organic ELelement 16 is connected to a low potential first voltage power sourceline Vss. The second capacitance Cx is formed of, for example, aparallel plate type capacitance, and is connected between the source ofthe second switch 26 and the gate of the driving transistor 22.

The image signal Data which is outputted from a signal conductor linedriving circuit (not shown) and which is formed of a voltage signal isinputted to each pixel circuit 18 via the signal conductor line X. Thepixel switch 20, the first switch 24, and the second switch 26 arerespectively controlled to be turned on and off by a control signal Sa,a control signal Sb, and a control signal Sc generated at a scanningline driving circuit (not shown).

In the third embodiment, since the second capacitance Cx is providedbetween the source of the second switch 26 and the gate of the drivingtransistor 22, the variation in the gate electric potential of thedriving transistor generated at the time of switching the first switch24 off can be compensated for. In accordance therewith, at the time ofcompleting Vth canceling operation, the influence of the variation orirregularity in the gate control voltage of the driving transistorcaused by a feedthrough voltage can be reduced, and the irregularity inbrightness among a plurality of display pixels can be suppressed.Accordingly, there can be obtained an active matrix type organic ELdisplay apparatus which is prevented from generating unevenness indisplaying, and in which the quality of displaying is improved, withoutincreasing the driving voltage and the size of the driving circuit andwithout increasing manufacturing cost thereof.

The present invention is not limited to the above-described embodiments,and the components can be modified and materialized within a range whichdoes not deviate from the gist of the present invention at the stage ofimplementing the invention. Further, various inventions can be formeddue to the plurality of components disclosed in the above-describedembodiments being appropriately combined. For example, some componentsmay be eliminated from all of the components shown in the embodiments.Moreover, the components ranging over the different embodiments may beappropriately combined.

In the embodiments described above, all of the thin-film transistorsconstituting the pixel circuits are formed of the same conductivity typetransistors, i.e., P-channel type transistors here. However, thethin-film transistors may be configured of N-channel type thin-filmtransistors. Further, the pixel circuit can be formed by includingdifferent conductive type thin-film transistors together such that,respectively, the pixel switch and the first switch are formed ofN-channel type thin-film transistors, and the driving transistor and thesecond switch are formed of P-channel type transistors, or the like.

The semiconductor layer of the thin-film transistor may be configuredof, not only polysilicon, but also amorphous silicon. Theself-luminescent element constituting the display element is not limitedto an organic EL element, and various luminescent elements which canemit light itself may be applied thereto.

1. An active matrix type display apparatus comprising: aself-luminescent element which is connected to a first voltage powersource line and which emits light in accordance with a supplied electriccurrent; a driving transistor which is connected between a secondvoltage power source line and the self-luminescent element and whichcontrols an electric current amount supplied to the self-luminescentelement in accordance with a gate control voltage; a first switch formedof a transistor and connected between a gate and a drain of the drivingtransistor; a first capacitance connected to the gate; a second switchformed of a transistor and connected between the drain of the drivingtransistor and the self-luminescent element; and a second capacitanceconnected between the second switch and the gate of the drivingtransistor.
 2. An active matrix type display apparatus according toclaim 1, wherein the second capacitance has a capacitance value in whicha sum of an amount of displacement in the gate control voltage generatedwhen the first switch is turned off and an amount of displacement in thegate control voltage generated when the second switch is turned on ismade to be substantially zero.
 3. An active matrix type displayapparatus according to claim 1, wherein the driving transistor, thefirst switch, and the second switch are respectively formed of thin-filmtransistors having semiconductor layers made of polysilicon.
 4. Anactive matrix type display apparatus according to claim 1, wherein thefirst switch and the second switch are respectively formed of thin-filmtransistors having the same conductivity, a drain of the second switchis connected to the self-luminescent element, a source of the secondswitch is connected to the drain of the driving transistor, and thesecond capacitance is connected between the source of the second switchand the gate of the driving transistor.
 5. An active matrix type displayapparatus according to claim 1, wherein the second capacitance is formedof a parallel plate capacitance.
 6. An active matrix type displayapparatus according to claim 1, wherein the second capacitance is formedof a parallel inter-wiring capacitance.
 7. An active matrix type displayapparatus according to claim 1, wherein the supplied electric current isset on the basis of an electric current signal.
 8. An active matrix typedisplay apparatus according to claim 1, wherein the self-luminescentelement has a pair of electrodes facing each other, and an organicemitting layer positioned between the electrodes.